Electrochemical machining of titanium and alloys thereof



United States Patent Office 3,409,522 ELECTROCHEMICAL MACHINING OF TITANIUM AND ALLOYS THEREOF John W. Grenier, Cincinnati, and Joseph Bayer, Middletown, Ohio, assignors to General Electric Company, a

corporation of New York No Drawing. Filed Dec. 10, 1965, Ser. No. 513,083 7 Claims. (Cl. 204143) This invention relates to aqueous electrolyte solutions and, more particularly, to an aqueous electrolyte particularly useful in the electrolytic machining of materials based on titanium.

The electrolytic machining of metals and alloys involves the use of a tool which is made cathodic .and a work-piece which is made anodic. Between the tool and workpiece there is placed an electrolyte which allows the current to pass between the tool and the workpiece and allows electrolytic machining to take place.

Electrolytes of various types have been used. For example, solutions of neutral salts, such .as aqueous solutions of sodium chloride, as well as highly acidic solutions, of which aqueous solutions of hydrochloric or sulfuric acids are typical, have been employed. Because of the corrosive effect highly acid electrolytes have upon the electrolytic machining equipment and because of the safety precautions which must be exercised by equipment operators, it is preferable to use .a relatively neutral electrolyte whenever possible.

The electrolytic machining of titanium and its alloys using substantially neutral electrolytes presents special problems which dictate the need for high electrical power requirements. In addition, the removal of the titanium with such known electrolytes leaves much to be desired in the surface finish obtained. One fundamental difficulty in the electrolytic machining of titanium and its alloys originates from the protective oxide film which forms on this highly reactive metal. This film, generated by exposure to moisture or air, is highly tenacious and insulative. As an oxide it possesses very few structural defects for the passage of elemental particles or electrons. The film is most perfect on commercially pure titanium, less perfect on titanium alloys. The starting voltages required to penetrate such oxide films at the start of and during electrolytic machining of titanium base materials with available electrolytes are appreciably higher than those required for such other alloys as the nickel base superalloys. Thus the achievable material removal rate is limited for a given power supply. In the case of commercially pure titanium, removal of the surface film with a preliminary etching solution has been used before the start of electrolytic machining.

Although relatively high starting voltages are needed to accomplish electrolytic machining of some alloys of titanium, the concentration of salt in the electrolyte must be held at a low level in order to obtain acceptable surface finishes. The low salt concentrations result in low electrolyte conductivity which further limits the metal removal rate achievable with a given power supply. Thus available electrolyte systems have not allowed the rapid and etficient electrolytic machining of titanium in a manner which results in a smooth desirable surface finish.

A principal object of this invention is to provide an improved electrolyte particularly useful for the electrolytic machining of titanium and its alloys which electrolyte has low power requirements and provides a smooth surface finish.

Another object is to provide such an aqueous electrolyte which is not corrosive to equipment.

These and other objects and advantages will be more clearly understood from the following detailed descrip- 3,409,522 Patented Nov. 5, 1968 tion and examples which are meant to be typical of rather than any limitation on the scope of the present invention.

It has been found that the above objects can be attained by combining in an aqueous solution, a chloride or bromide or both of water soluble substantially neutral salts with a water soluble complexing agent which will combine with the titanium ion being formed at the anode during electrolytic machining. In particular, the electrolyte of the present invention is an improved aqueous electrolyte consisting essentially of about 1-4 molar concentration of water soluble compounds of Group IA elements (alkali metals) and elements selected from the group consisting of chlorine and bromine and a complexing agent which will combine with the titanium ion being formed at the anode.

In one form, the improved aqueous electrolyte of the present invention is an aqueous solution consisting essentially of about 1-4 molar concentration of water soluble compounds selected from the group consisting of NaCl and KBr and a water soluble complexing agent selected from the group consisting of sodium citrate and a water soluble salt of ethylenediaminetetraiicetic acid, the sodium citrate when selected being included at a concentration of less than 0.5 molar and the ethylenediaminetetraacetic acid salt when selected being included up to the amount required to saturate the solution.

As was indicated above, titanium and its alloys present special problems because all titanium salts hydrolyze in water to form an insoluble oxide. Furthermore, aluminum and vanadium, frequently used as alloying elements in titanium alloys, exhibit similar behavior. The formation of the oxide would not be a particularly awkward problem were it not for the fact that these oxides tend to produce adherent, insulative films that are diificult to penetrate by ions during electrolytic machining. Avoidance of the hydrolysis product can only be achieved by sufficient acidity in the electrolyte to supress the reaction, assuming that a soluble specie is formed upon dissolution of the anode. The present invention, however, is directed to another means for preventing the formation of insulative oxide buildup without the use of highly acid solutions. This can be accomplished by interaction of the titanium ion with such agents as water soluble citrates and the water soluble salts of ethylenediaminetetraacetic acid at the interface film.

Typical of the wide variety of aqueous solutions studied in the evaluation of the present invention are those presented in Table I.

TABLE I.AQ,UEOUS SOLUTION EFFECT ON 6% A1, 4% V,

BAL. Ti ALLOY NaCl Na3C H O Disodium Current Surface Ex. (molar) (molar) EDTA. Density Condition (molar) (amps/em.])

1 1.0 0.1 64 Fair. 2.. 1. 0 0. 1 D0. 3.. 1.0 1. 5 Very poor. 4.. 1. 5 0. 5 64 Poor. 5.. 1. 5 0. 5 130 D0. 6.. 2.0 1. 0 64 Very poor 7.. 2.0 1.0 D0 8.. 3.0 0. 1 64 Very good 9.. 3. 0 0. 1 130 Do 10. 1. 0 0. 1 64 Fair 11. 1. 0 0. 1 130 Do. 12..- 2.0 1.0 Very poor. 13. 3. 9 0 64 Excellent. 14. 3. 9 0 130 Do.

a Erratic current flow due to anodic film formation. l tsolution saturated with ethylenediamlnetetraacetic acid disodium sa In Table I, EDTA refers to ethylenediaminetetaacetic acid. In these examples, the disodium salt was used although it should be understood that any of the water soluble salts of EDTA can be used as well.

As shown by Examples 3, 4, 5, 6, 7 and 12, the inclusion of 0.5 or more molar concentration of the trisotlium citrate with about 1.4. molar. concentrationofsodium chloride results in a rough, unsatisfactory surface on the 6% Al, 4% V, balance Ti alloy. It was recognized that at molar concentrations of less than 0.5 molar trisodium citrate, for example, at 0.1 molar trisodium citrate, significant improvement in surface finish can be obtained along with a good rate of material removal at relatively low power input.

The best surface finishes, which were almost specular in quality, were obtained with the electrolyte of Examples 13 and 14 in which the 3.9 molar aqueous solution of sodium chloride was saturated with the disodium salt of EDTA. Highly satisfactory finishes were obtained with thesolutions of Examples 8 and 9 almost comparable withthe results obtained with Examples 13 and 14.

In Examples 3, 4, 5, 6, 7 and 12, in which the trisodium citrate concentration was relatively high and outside the range of the present invention, the anodic film was produced very rapidly causing an abrupt drop in current. In these instances, very little metal was removed from the anode. The anode surfaces were rough and pitted and the anodic film which was formed contained large amounts of insoluble salts and other insoluble materials. Thus in the specifically preferred form of the present invention sodium chloride is included at a molar concentration of about 1-4 the trisodium citrate when selected as the complexing agent is included at a molar concentration of less than 0.5 and the soluble sodium salt of EDTA is included in an amount up to that which will saturate the aqueous electrolyte.

The following Table II presents other characteristics of the aqueous solutions shown in Table I.

TABLE II.AQUEOUS SOLUTION DATA TABLE III.AQ,UEOUS SOLUTION EFFECT ON PURE TITANIUM NaCl KBr Na;CH5O Polarization Surface Ex. (molar) (molar) (molar) Potential Finish (volts) 18 Ridges.

6 Very rough. 7 Irregular. 7 Smooth.

As shown by Example 15, the polarization potential for a 3 molar NaCl aqueous solution is unusually high and results in a surface having unsatisfactory ridges. KBr at about the same concentration results in a lower polarization potential. However, a rough surface results. This surface finish at these concentrations is not assisted by the intermixture of sodium chloride and potassium bromide, as shown in Example 17. However, the addition of trisodium citrate at a 0.1 molar concentration results in a smooth surface finish.

As a result of the use of a complexing agent which will combine with titanium ion at the anode, a fine easily separable, crystalline precipitate forms. Through the use of an aqueous solution of sodium chloride alone, a sludge is produced in addition to the unsatisfactory surface condition on the workpiece.

The aqueous solution of the present invention lies within the pH range of 5-7. Therefore, the electrolyte of the present invention provides the added benefit of being significantly less corrosive to equipment than would be the highly acidic electrolytes required to inhibit the formation of the titanium oxide film on the titanium workpiece.

During evaluation of the present invention, other electrolytes including materials chemically similar to those of this invention were tested. The following Table IV gives the lists of such electrolytes, the conditions under which they were tested and the surface condition which results.

TABLE IV.OTHER AQUEOUS ELECTROLYTES Example Electrolyte Anode Volts Amps Surface Condition M tartaric acid T1 24 0.13 Heavily pitted, irregular.

24 12 Heavily pitted. T -GAI-4V. 20 0.35 Irregular, much oxide on surface. Ti6Al4V Fully insulated film formed. 0.25 M acetic acid, sodium acetate, 3. M

NaCl T1 5. 5. Irregular. 24 0.25 M acetic acid, sodium acetate, 3. M

NaCl Ti 20 D 25 4 M NaCl, .5 M citric acid Ti6Al4V.- l7 1 Ripples in Surface.

( Sodium acetate added in an amount to buffer to pH of 5.

In the tests represented by Tables I and II, each titanium alloy anode was prepared by cutting approximately /8" long segments from a 0.250 diameter rod. Each specimen was polished on one face first with number grit paper and then 2/0 emery cloth. After cleaning and drying, each specimen was weighed. Following each test run, the weight loss of the specimen and the coulometric output or number of Faradays of the run was determined. The surface finish was evaluated by examination in a biocular microscope at 250X. The valency was calculated from the weight loss and the electrical current consumption assuming an atomic weight of 47.9 for the titanium 6Al4V alloy. Test runs were made at two levels of current density for each electrolyte system. The duration The examples of the above table show that such materials as acetic, tartaric and oxalic acids do not result in the improved surface condition which can be achieved through the use of the complexing agents of the present invention as shown in the above Tables I and III. In addition, Examples 20 and 21 show that other halide salts such as the fluorides and other commonly used water soluble Group I-A salts such as KNO do not function as do the chlorides and bromides for the purposes of this invention. It is to be noted in Example 25 that the higher concentration of citric acid combined with sodium chloride does not result in a satisfactory surface condition.

Although the invention has been described in connection with certain specific examples, it will be recognized by those skilled in the art the variations and modifications of which this invention is capable. It is intended by the appended claims to cover all such equivalents.

What is claimed is:

1. In the electrolytic material removal process of metallic materials based on titanium, said removal process including disposing a tool opposite said titanium based materials and spaced therefrom to provide a machining gap, causing electrolyte to flow rapidly through said gap, and connecting said titanium based materials to a source of electric current so as to make said materials predominantly anodic whereby dissolution of said titanium based materials takes place, the improvement wherein the electrolyte has a pH of 5-7 and consists essentially of:

about l-4 molar concentration of water soluble compounds selected from the group consisting of alkali metal salts of chlorides and bromides, and

a water soluble complexing agent selected from the group consisting of sodium citrate and water soluble salt of ethylenediaminetetraacetic acid, the sodium citrate when selected being included at a concentration of less than 0.5 molar and the ethylenediaminetetraacetic acid salt when selected being included up to the amount required to saturate the electrolyte.

2. The process of claim 1 in which:

the water soluble compounds are in the range of about 1-4 molar concentration, and

the complexing agents consist essentially of 0.1 to less than 0.5 molar sodium citrate and up to the concentration of a water soluble sodium salt of ethylenediaminetetraacetic acid to saturate the electrolyte.

3. The process of claim 1 consisting essentially of about 1-4 molar NaCl, up to about 0.1 molar sodium citrate and from about 0.1 molar up to that concentration of a sodium salt of ethylenediaminetetraacetic acid required to saturate the electrolyte.

4. The process of claim 1 consisting essentially of 14 molar NaCl and from 0.1 molar to less than 0.5 molar sodium citrate.

5. The process of claim 1 consisting essentially of about 4 molar NaCl and an amount of the disodium salt of ethylenediaminetetraacetic acid to saturate the electrolyte.

6. The process of claim 1 consisting essentially of about 3 molar NaCl and about 0.1 molar sodium citrate.

7. The process of claim 1 consisting essentially of about 1.5 molar NaCl, about 1.5 molar Km and about 0.1 molar sodium citrate.

References Cited UNITED STATES PATENTS 2,939,825 6/1960 Faust et a1 204-142 ROBERT K. MIHALEK, Primary Examiner. 

1. IN THE ELECTROLYTIC MATERIAL REMOVAL PROCESS OF METALLIC MATERIALS BASED ON TITANIUM, SAID REMOVAL PROCESS INCLUDING DISPOSING A TOOL OPPOSITE SAID TITANIUM BASED MATERIALS AND SPACED THEREFROM TO PROVIDE A MACHINING GAP, CAUSING ELECTROLYTE TO FLOW RAPIDLY THROUGH SAID GAP, AND CONNECTING SAID TITANIUM BASED MATERIALS TO A SOURCE OF ELECTRIC CURRENT SO AS TO MAKE SAID MATERIALS PREDOMINANTLY ANODIC WHEREBY DISSOLUTION OF SAID TITANIUM BASED MATERIALS TAKES PLACE, THE IMPROVEMENT WHEREIN THE ELECTROLYTE HAS A PH OF 5-7 AND CONSISTS ESSENTIALLY OF: ABOUT 1-4 MOLAR CONCENTRATION OF WATER SOLUBLE COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL SALTS OF CHLORIDES AND BROMIDES, AND A WATER SOLUBLE COMPLEXING AGENT SELECTED FROM THE GROUP CONSISTING OF SODIUM CITRATE AND WATER SOLUBLE SALT OF ETHYLENEDIAMINETETRAACETIC ACID, THE SODIUM CITRATE WHEN SELECTED BEING INCLUDED AT A CONCENTRATION OF LESS THAN 0.5 MOLAR AND THE ETHYLENEDIAMINETETRAACETIC ACID SALT WHEN SELECTED BEING INDLUDED UP TO THE AMOUNT REQUIRED TO SATURATE THE ELECTROLYTE. 